Twenty-one people have received medical treatment in connection with this week's nerve agent attack on former Russian intelligence officer Sergei Skripal and his daughter Yulia in southern England.

The Skripals and a police officer who arrived on the scene to respond to the incident are included in the count, Wiltshire Police said Thursday, according to the Press Association.

Skripal – a former colonel in Russian intelligence who was imprisoned in Russia in 2006 on allegations of spying for Britain – was found unconscious with his daughter on a bench near a shopping center in the city of Salisbury on Sunday.

The poisoning of father and daughter has drawn media speculation that Russian state actors could be behind the attack, prompting comparisons to other suspicious deaths of Russians on British soil – specifically Alexander Litvinenko in 2006 and Alexander Perepilichny in 2012.

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Litvinenko, another Russian former intelligence officer, died after drinking tea laced with a radioactive isotope. A British inquiry concluded that he was targeted for assassination by Russian agents.

Perepilichny, who collapsed and died outside his home while jogging in southern England in 2012, had reported death threats against him following his accusations of large-scale tax fraud by Russian officials.

Skripal, 66, and his daughter, 30, remained unconscious and in critical but stable condition on Thursday, Home Secretary Amber Rudd told the House of Commons.

The officer, identified by police as Detective Sergeant Nick Bailey, was "talking and engaging" but remains in serious condition, Rudd said.

"I would like to pay tribute to all of the emergency services in Salisbury – those who reacted to the initial call on Sunday and those who continue to respond to this appalling and reckless attack," Prime Minister Theresa May said.

"The events of Sunday are a stark reminder, if ever one was needed, of the dangerous situations our emergency services face and the dedication and courage they display every day in order to keep us safe," May sais.

Earlier on Thursday, Rudd told BBC Radio 4's "Today" program that poison used on Skripal was a "very rare" nerve agent, calling the use of the substance in Britain a "brazen and reckless act."

The broadcaster later cited an unidentified source as saying the nerve agent was "likely to be rarer" than sarin and was "said not to be VX" – the chemical weapon used to kill Kim Jong Nam, the half brother of North Korean leader Kim Jong Un, in Malaysia last year.

The Telegraph reported Wednesday that Skripal "became close to" a British security consultant who also lived in Salisbury and worked for a company owned by Christopher Steele, the author of a controversial dossier on U.S. President Donald Trump's alleged links to Russia.